1. Field of the Invention
The present invention relates generally to gripping mechanisms for downhole tools.
2. Description of the Related Art
Tractors for moving within underground boreholes are used for a variety of purposes, such as oil drilling, mining, laying communication lines, and many other purposes. In the petroleum industry, for example, a typical oil well comprises a vertical borehole that is drilled by a rotary drill bit attached to the end of a drill string. The drill string may be constructed of a series of connected links of drill pipe that extend between ground surface equipment and the aft end of the tractor. Alternatively, the drill string may comprise flexible tubing or “coiled tubing” connected to the aft end of the tractor. A drilling fluid, such as drilling mud, is pumped from the ground surface equipment through an interior flow channel of the drill string and through the tractor to the drill bit. The drilling fluid is used to cool and lubricate the bit, and to remove debris and rock chips from the borehole, which are created by the drilling process. The drilling fluid returns to the surface, carrying the cuttings and debris, through the annular space between the outer surface of the drill pipe and the inner surface of the borehole.
Tractors for moving within downhole passages are often required to operate in harsh environments and limited space. For example, tractors used for oil drilling may encounter hydrostatic pressures as high as 16,000 psi and temperatures as high as 300° F. Typical boreholes for oil drilling are 3.5-27.5 inches in diameter. Further, to permit turning, the tractor length should be limited. Also, tractors must often have the capability to generate and exert substantial force against a formation. For example, operations such as drilling require thrust forces as high as 30,000 pounds.
As a result of the harsh working environment, space constraints, and desired force generation requirements, downhole tractors are used only in very limited situations, such as within existing well bore casing. While a number of the inventors of this application have previously developed a significantly improved design for a downhole tractor, further improvements are desirable to achieve performance levels that would permit downhole tractors to achieve commercial success in other environments, such as open bore drilling.
Western Well Tool, Incorporated has developed a variety of downhole tractors for drilling, completion and intervention processes for wells and boreholes. For example, the Puller-Thruster Tractor is a multi-purpose tractor (U.S. Pat. Nos. 6,003,606, 6,286,592, and 6,601,652) that can be used in rotary, coiled tubing and wireline operations. A method of moving is described in U.S. Pat. No. 6,230,813. The Electro-hydraulically Controlled Tractor (U.S. Pat. Nos. 6,241,031 and 6,427,786) defines a tractor that utilizes both electrical and hydraulic control methods. The Electrically Sequenced Tractor (U.S. Pat. No. 6,347,674) defines a sophisticated electrically controlled tractor. The Intervention Tractor (also called the Tractor with improved valve system, U.S. Pat. No. 6,679,341 and U.S. Patent Application Publication No. 2004/0168828) is preferably an all hydraulic tractor intended for use with coiled tubing that provides locomotion downhole to deliver heavy loads such as perforation guns and sand washing.
These various tractors are intended to provide locomotion, to pull or push various types of loads. For each of these various types of tractors, various types of gripper elements have been developed. Thus an important part of the downhole tractor tool is its gripper system.
In one known design, a tractor comprises an elongated body, a propulsion system for applying thrust to the body, and grippers for anchoring the tractor to the inner surface of a borehole or passage while such thrust is applied to the body. Each gripper has an actuated position in which the gripper substantially prevents relative movement between the gripper and the inner surface of the passage, and a retracted position in which the gripper permits substantially free relative movement between the gripper and the inner surface of the passage. Typically, each gripper is slidingly engaged with the tractor body so that the body can be thrust longitudinally while the gripper is actuated. The grippers preferably do not substantially impede “flow-by,” the flow of fluid returning from the drill bit up to the ground surface through the annulus between the tractor and the borehole surface.
Tractors may have at least two grippers that alternately actuate and reset to assist the motion of the tractor. In one cycle of operation, the body is thrust longitudinally along a first stroke length while a first gripper is actuated and a second gripper is retracted. During the first stroke length, the second gripper moves along the tractor body in a reset motion. Then, the second gripper is actuated and the first gripper is subsequently retracted. The body is thrust longitudinally along a second stroke length. During the second stroke length, the first gripper moves along the tractor body in a reset motion. The first gripper is then actuated and the second gripper subsequently retracted. The cycle then repeats. Alternatively, a tractor may be equipped with only a single gripper for specialized applications of well intervention, such as movement of sliding sleeves or perforation equipment.
Grippers may be designed to be powered by fluid, such as drilling mud in an open tractor system or hydraulic fluid in a closed tractor system. Typically, a gripper assembly has an actuation fluid chamber that receives pressurized fluid to cause the gripper to move to its actuated position. The gripper assembly may also have a retraction fluid chamber that receives pressurized fluid to cause the gripper to move to its retracted position. Alternatively, the gripper assembly may have a mechanical retraction element, such as a coil spring or leaf spring, which biases the gripper back to its retracted position when the pressurized fluid is discharged. Motor-operated or hydraulically controlled valves in the tractor body can control the delivery of fluid to the various chambers of the gripper assembly.
The original design of the Western Well Tool Puller-Thruster Tractor incorporated the use of an inflatable reinforced rubber packer (i.e., “Packerfoot”) as a means of anchoring the tool in the well bore. This original gripper concept was improved with various types of reinforcement in U.S. Pat. No. 6,431,291, entitled “Packerfoot Having Reduced Likelihood of Bladder Delamination.” This concept developed a “Gripper” with an expansion diameter of approximately 1 inch. This design was susceptible to premature failure of the fiber terminations, subsequent delamination and pressure boundary failure. The second “Gripper” concept was the Roller Toe Gripper (U.S. Pat. No. 6,464,003). The current embodiment of this gripper works exceedingly well, however in one current embodiment, there are limits to the extent of diametrical expansion, thus limiting the well bore variations compatible with the “Gripper” anchoring. Historically, the average diametrical expansion has averaged approximately 2 inches.
Additionally, The prior art includes a variety of different types of grippers for tractors. One type of gripper comprises a plurality of frictional elements, such as metallic friction pads, blocks, or plates, which are disposed about the circumference of the tractor body. The frictional elements are forced radially outward against the inner surface of a borehole under the force of fluid pressure. However, these gripper designs are either too large to fit within the small dimensions of a borehole or have limited radial expansion capabilities. Also, the size of these grippers often cause a large pressure drop in the flow-by fluid, i.e., the fluid returning from the drill bit up through the annulus between the tractor and the borehole. The pressure drop makes it harder to force the returning fluid up to the surface. Also, the pressure drop may cause drill cuttings to drop out of the main fluid path and clog up the annulus.
Another type of gripper comprises a bladder that is inflated by fluid to bear against the borehole surface. While inflatable bladders provide good conformance to the possibly irregular dimensions of a borehole, they do not provide very good torsional resistance. In other words, bladders tend to permit a certain degree of undesirable twisting or rotation of the tractor body, which may confuse the tractor's position sensors. Additionally, some bladder configurations have durability issues as the bladder material may wear and degrade with repeated usage cycles. Also, some bladder configurations may substantially impede the flow-by of fluid and drill cuttings returning up through the annulus to the surface.
Yet another type of gripper comprises a combination of bladders and flexible beams oriented generally parallel to the tractor body on the radial exterior of the bladders. The ends of the beams are maintained at a constant radial position near the surface of the tractor body, and may be permitted to slide longitudinally. Inflation of the bladders causes the beams to flex outwardly and contact the borehole wall. This design effectively separates the loads associated with radial expansion and torque. The bladders provide the loads for radial expansion and gripping onto the borehole wall, and the beams resist twisting or rotation of the tractor body. While this design represents a significant advancement over previous designs, the bladders provide limited radial expansion loads. As a result, the design is less effective in certain environments. Also, this design impedes to some extent the flow of fluid and drill cuttings upward through the annulus.
Some types of grippers have gripping elements that are actuated or retracted by causing different surfaces of the gripper assembly to slide against each other. Moving the gripper between its actuated and retracted positions involves substantial sliding friction between these sliding surfaces. The sliding friction is proportional to the normal forces between the sliding surfaces. A major disadvantage of these grippers is that the sliding friction can significantly impede their operation, especially if the normal forces between the sliding surfaces are large. The sliding friction may limit the extent of radial displacement of the gripping elements as well as the amount of radial gripping force that is applied to the inner surface of a borehole. Thus, it may be difficult to transmit larger loads to the passage, as may be required for certain operations, such as drilling. Another disadvantage of these grippers is that drilling fluid, drill cuttings, and other particles can get caught between and damage the sliding surfaces as they slide against one another. Also, such intermediate particles can add to the sliding friction and further impede actuation and retraction of the gripper.
Yet another type of gripper comprises a pair of four-bar linkages separated by 180° about the circumference of the tractor body. FIG. 14 shows such a design. Each linkage 200 comprises a first link 201, a second link 203, and a third link 205. The first link 201 has a first end 207 pivotally or hingedly secured at or near the surface of the tractor body 209, and a second end 211 pivotally secured to a first end 213 of the second link 203. The second link 203 has a second end 215 pivotally secured to a first end 217 of the third link 205. The third link 205 has a second end 219 pivotally secured at or near the surface of the tractor body 209. The first end 207 of the first link 201 and the second end 219 of the third link 205 are maintained at a constant radial position and are longitudinally slidable with respect to one another. The second link 203 is designed to bear against the inner surface of a borehole wall. Radial displacement of the second link 203 is caused by the application of longitudinally directed fluid pressure forces onto the first end 207 of the first link 201 and/or the second end 219 of the third link 205, to force such ends toward one another. As the ends 207 and 219 move toward one another, the second link 203 moves radially outward to bear against the borehole surface and anchor the tractor.
One major disadvantage of the four-bar linkage gripper design is that it is difficult to generate significant radial expansion loads against the inner surface of the borehole until the second link 203 has been radially displaced a substantial degree. As noted above, the radial load applied to the borehole is generated by applying longitudinally directed fluid pressure forces onto the first and third links. These fluid pressure forces cause the first end 207 of the first link 201 and the second end 219 of the third link 205 to move together until the second link 203 makes contact with the borehole. Then, the fluid pressure forces are transmitted through the first and third links to the second link and onto the borehole wall. However, the radial component of the transmitted forces is proportional to the sine of the angle θ between the first or third link and the tractor body 209. In the retracted position of the gripper, all three of the links are oriented generally parallel to the tractor body 209, so that θ is zero or very small. Thus, when the gripper is in or is near the retracted position, the gripper may be incapable of transmitting significant radial load to the borehole wall. In boreholes, in which the second link 203 is displaced only slightly before coming into contact with the borehole surface, the gripper provides a very limited radial load compared to the longitudinal force exerted. Thus, in small diameter environments, the gripper may not be able to reliably anchor the tractor. The gripping ability of the four bar linkage improves significantly, however, as the angle θ reaches approximately 50° and above. As a result, this four-bar linkage gripper may not be useful in small diameter boreholes or in small diameter sections of generally larger boreholes. If the four-bar linkage was modified so that the angle θ is always large, the linkage may then be able to accommodate only very small variations in the diameter of the borehole.